Google today celebrated Chrome's second birthday by launching the sixth version of its browser for Windows, Mac and Linux.

Chrome 6, which Google released into its "stable" channel -- the upgrade mechanism for the production-quality version -- also included patches for 16 vulnerabilities and another crack at fixing a Windows kernel bug that affected the browser.

"The last couple of years we've been focused on speed," said Brian Rakowski, Chrome's director of product management. "A lot of things have changed in the last two years [in browsers], but the one thing we've learned is that speed matters. It's something Google's always believed in and it resonates with people."

One analyst isn't so sure. "Speed is absolutely important," said Sheri McLeish of Forrester Research. "But it's really just a horse race, with whoever comes out with the latest release generally the fastest."

Google introduced Chrome on Sept. 2, 2008, and set rivals scrambling to match its speed, particularly in JavaScript rendering, and then later, to mimic its minimalist user interface (UI) design.

Chrome 6 is three times faster at parsing JavaScript than the 2008 debut, Rakowski claimed.

"That's a pretty big deal," Rakowski said, "but we have a lot more speed improvements to come."

Hardware acceleration is one performance enhancer that Google is just beginning to show to developers. Rakowski promised end users would see it soon.

Browser hardware acceleration shifts some tasks from a PC's main processor to its graphics processor to boost performance, especially of graphics-intensive chores like rendering video or complex three-dimensional objects. So far, Google has included it only with developer builds of the open-source Chromium project, but the company will begin feeding the feature to Chrome -- first to the browser's dev channel, then as is its normal practice -- to the beta and stable builds.

Chrome for Windows and Mac will both see hardware acceleration, although through different technologies. Rakowski was unsure whether the Linux edition would see the feature as well.

Chrome 6 also sports a minor UI makeover, with some elements shifted -- the bookmark icon has been moved to the right of the address bar -- and others compressed. Chrome now sports a single menu, down from two earlier, that hides all but the most basic browsing command, such as page forward, page backward and page reload.

"Chrome did up the ante," agreed McLeish, "and kept others on their toes. They simplified the user experience, changing the browser interface to be consistent with the Google brand."

"We've been known for our simplified UI," said Rakowski. "And we'll continue to clean up the visuals."

Chrome 6 also boasts enhanced sync that lets users synchronize extensions and autofill entries between copies of the browser on different machines. Previously, the tool synced bookmarks, passwords, preferences and themes only.

Google plans to pick up the upgrade pace by releasing new versions of Chrome every six weeks or so. "We'll be updating much more frequently to a more regular and quick cadence," Rakowski said.

At that pace, Chrome will refresh significantly faster that its rivals. Mozilla's Firefox, for example, issued just one major update during 2009, and hopes to up that to two releases this year. Microsoft's IE, meanwhile, refreshes about once every two years.

Chrome 6 included fixes for 16 security vulnerabilities, eight of them rated "high," four as "medium," and four "low." None were ranked "critical," Google's highest threat indicator in its four-step scoring system. Google paid out $4,337 in bounties for six of the 16 bugs.

The browser also included a second fix for a Windows kernel problem that was declared solved in mid-August.

Chrome has already met the goal the browser's engineering director set a year ago when he said Chrome would own 5% of the market by September 2010, and 10% by September 2011. According to the latest statistics from Web metrics company Net Applications, Chrome had a 7.5% share in August.

But McLeish was unimpressed with Chrome's progress. "I wouldn't call that an overwhelming success," she said. "They've been aggressive in their expectations, but 7% in two years is rather slow for something the size of Google."

Not surprisingly, Rakowski saw it differently. "We think the [adoption] pace has been astounding," he said, adding that Chrome now has some 70 million users worldwide. "We've been really happy to be highly regarded by the people in the know. The next challenge is breaking into the mainstream."

Chrome 6 can be downloaded for Windows, Mac OS X and Linux from Google's Web site. Users already running the browser will be updated automatically.